A transformer is a device that transfers electrical energy from one circuit to another by magnetic coupling. Typically, a transformer includes one or more windings wrapped around a core. An alternating voltage applied to one winding (a “primary winding”) creates a time-varying magnetic flux in the core, which induces a voltage in the other (“secondary”) winding(s). Varying the relative number of turns of the primary and secondary windings about the core determines the ratio of the input and output voltages of the transformer. For example, a transformer with a turn ratio of 2:1 (primary:secondary) has an input voltage that is two times greater than its output voltage.
A transformer tap is a connection point along a transformer winding that allows the number of turns of the winding to be selected. Thus, a transformer tap enables a transformer to have variable turn ratios. Selection of the turn ratio in use is made by operating a tap changer switch. For simplicity, the term “switch” is used herein to refer to a tap changer switch. Popular turns ratios have evolved and have been standardized. One such standard is the dual voltage transformer that includes two windings which can be connected in series to handle a specified voltage and amperage, or in parallel to handle double the amperage at one half the series connected voltage.
Typical tap changer switch designs have also evolved to support the most popular standard turns ratios. For instance, a “dual voltage” switch is configured specifically for connection to the tap arrangement of a dual voltage transformer. Whereas a traditional switch has connection points for six taps of the transformer winding, a dual voltage switch has only four connection points.
Another typical switch in the art is a “multi-deck” switch that is created by stacking and connecting two or more tap changer switches together. The switches in the stack are all interconnected in such a way as to prevent independent operation. A multi-deck switch is employed for transformer winding configurations that have more taps than can be satisfied by one switch.
It is well known in the art to cool high-power transformers using a dielectric fluid, such as a highly-refined mineral oil. The dielectric fluid is stable at high temperatures and has excellent insulating properties for suppressing corona discharge and electric arcing in the transformer. Typically, the transformer includes a tank that is at least partially filled with the dielectric fluid. The dielectric fluid surrounds the transformer core and windings.
A core clamp extends from the core and maintains the relative positions of the core and the windings in the tank. A switch is mounted to a side wall of the tank. The switch includes one or more decks electrically coupled to at least one of the windings, for altering a voltage of the transformer.
Metallic screws and non-metallic bars are used to fasten the switch decks together in conventional multi-deck switches. The screws, while not electrically live, are conductive. Therefore, the screws can act to reduce electrical clearance between the switch contacts and the grounded tank wall and core clamp. To meet minimum electrical clearance to ground requirements, there must be at least a minimum distance between the live contacts, screws, and grounded tank wall and core clamp.
Minimum electrical clearances are required between the electrical contacts in the adjacent decks of a multi-deck switch. The bars that connect the decks together produce the distances between contacts that are necessary to comply with clearance requirements.
As the size of the switch increases, the tank must get wider or the switch must be mounted above the core clamp, in a taller tank, to meet the minimum distance requirement. As the size of the tank increases, the cost of acquiring and maintaining the transformer increases. For example, a larger transformer requires more space and more tank material. The larger transformer also requires more dielectric fluid to fill the transformer's larger tank. Thus, the cost of the transformer is directly proportional to the size of the switch.
Therefore, a need exists in the art for a switch having a decreased size. In addition, a need exists in the art for a switch with increased electrical clearance with the grounded tank wall. A further need exists in the art for a switch devoid of metallic screws for fastening the switch decks of a multideck switch, together.